The present invention generally relates to image forming apparatuses, and more particularly to an image forming apparatus including a plurality of latent image bearing members on which latent images are formed and successively transferring images of different colors formed on the latent image bearing members onto the same transfer member such as a sheet of paper.
Conventionally, in a laser scan type recording apparatus which uses an optical scan system, a laser beam detector detects a scan start position of a laser beam, and reading of pixel information is started after receiving an output of the laser beam detector. The laser beam is modulated using the pixel information and a pixel clock, and the modulated laser beam is irradiated on a photoconductive body so as to form a toner latent image on the photoconductive body. The toner latent image is developed, and an image formation is made by transferring the developed image onto a recording medium.
The output of the laser beam detector and the pixel clock are physically not synchronized. For this reason, a deviation amounting to a maximum of 1 pixel clock may occur in the modulation start timing of the laser beam, thereby causing a pixel shear (or shift). Hence, there was a conventional image forming apparatus which detects the output of the laser beam detector using a clock having N times the frequency of the pixel clock, and then obtaining the pixel clock by frequency-dividing the detected output by 1/N. On the other hand, there was another conventional image forming apparatus which provides pixel clocks having N phases and detection circuits amounting to the number of phases, and obtaining the pixel clock based on the first clock phase detected from the output of the laser beam detector.
However, the conventional image forming apparatuses had the following problems to be solved.
First, in the laser scan system using a rotary polygonal mirror, an image scan width differed for each face of the polygonal mirror due to the different characteristics of the faces, such as the face irregularities or inclinations, the f.THETA. characteristics, the focal distances and the uniformness of velocities. As a result, with respect to an image on the scan ending side, a periodic image shear occurred in a sub scan direction which is perpendicular to a main scan direction depending on the number of faces of the polygonal mirror.
Second, in the image forming apparatus which arranges a plurality of recording units for the different colors and forms a multi-color image on the recording medium such as paper by overlapping transfers of different color pixel data onto the recording medium, the position of the laser beam detector and the characteristics of the polygonal mirror differ for each recording unit, and the causes for the image shear further increased.